WO2019128741A1 - Electron transport material and preparation method therefor, and quantum dot light emitting diode - Google Patents

Abstract

An electron transport material and a preparation method therefor, and a quantum dot light emitting diode. The electron transport material is a metal ion doped zinc oxide, the metal ions being two or three metal ions of the same metal element in different valence states, the lowest valence state of the metal lines being positive divalent. The present electron transport material uses ions of the same metal element in high and low valences to co-dope the zinc oxide material, the doping of the zinc oxide material by means of the low valence metal ions obviously increasing the doping limit of the high valence metal ions in the zinc oxide material, significantly improving the electrical conductivity of the zinc oxide electron transport layer and ultimately improving the light emitting efficiency of the device.

Description

Electron transport material, preparation method thereof and quantum dot light emitting diode Technical field

The invention belongs to the technical field of materials, and in particular relates to an electron transporting material, a preparation method thereof and a quantum dot light emitting diode.

Background technique

Recently, with the continuous development of display technology, quantum dot light-emitting diodes using quantum dot materials as light-emitting layers (Quantum Dot) Light Emitting Diode (QLED) is getting more and more people's attention. It has high luminous efficiency, controllable color, high color purity, good device stability, and can be used for flexible applications. QLED is used in display technology, solid-state lighting, etc. The field has great application prospects.

As the electron transport layer material commonly used in quantum dot light-emitting diodes, zinc oxide has a good energy level matching relationship with the cathode and quantum dot light-emitting layer, which can significantly reduce the injection barrier of electrons from the cathode to the quantum dot light-emitting layer. And its deeper valence band energy level can also function to effectively block holes. In addition, the zinc oxide material has excellent electron transport capability and its electron mobility is as high as 10 ^-3 cm ² /V·S. These characteristics make the zinc oxide material the first material of the electron transport layer in the quantum dot light emitting diode device, which can significantly improve the stability and luminous efficiency of the device.

In order to further improve the device performance of the quantum dot light emitting diode, a method in which a metal ion is doped with a zinc oxide electron transport layer is widely used. The method can adjust the forbidden band width of the zinc oxide electron transport layer to a certain extent, optimize the conductivity and energy level structure of the zinc oxide electron transport layer, and thereby improve the luminous efficiency of the quantum dot light emitting diode. For example, doping a zinc oxide material with a high-valent metal ion such as Al ³⁺ or Ga ³⁺ can increase the free carrier density of the zinc oxide material and obtain excellent conductivity performance. In order to further increase the electrical conductivity of the zinc oxide material, it is necessary to increase the amount of the dopant. However, due to the large ionic radius difference between the commonly used high-valent metal ions and Zn ²⁺ , this greatly limits the solid solubility of the high-valent metal ions in the zinc oxide, making it difficult to further improve the electrical conductivity of the zinc oxide material. Therefore, the prior art has yet to be improved and developed.

technical problem

The object of the present invention is to provide an electron transporting material, a preparation method thereof and a quantum dot light emitting diode, which aim to solve the problem that the solid solubility of the existing high-valent metal ion doped zinc oxide is limited, so that the conductive property of zinc oxide is difficult to be improved. Therefore, the luminous efficiency of the quantum dot light emitting diode is not satisfactory.

Technical solution

In order to achieve the above object, the technical solution adopted by the present invention is as follows:

An aspect of the invention provides an electron transporting material, which is metal ion doped zinc oxide, the metal ions being two or three metal ions of different valence states of the same metal element, and the The lowest valence state of metal ions is positive divalent.

Correspondingly, a method for preparing an electron transporting material comprises the following steps:

Providing a zinc salt and a divalent doped metal salt, dissolving the zinc salt and the divalent doped metal salt in a first solvent, and obtaining divalent metal ion doped zinc oxide nanoparticles under alkaline conditions;

Dispersing the divalent metal ion doped zinc oxide nanoparticles in a second solvent to obtain a divalent metal ion doped zinc oxide colloid solution;

Depositing the divalent metal ion doped zinc oxide colloid solution on the substrate and performing an oxidation treatment to obtain the electron transporting material doped with metal ions, wherein the metal ions are two different valence states of the same metal element Or three metal ions, and the lowest valence state of the metal ions is positive divalent.

Another aspect of the present invention provides a quantum dot light emitting diode comprising an anode, a cathode, and a quantum dot emitting layer between the anode and the cathode, and an electron transport is disposed between the cathode and the quantum dot emitting layer The layer, the electron transport layer, is composed of the above-described electron transport material of the present invention.

Correspondingly, a method for preparing a quantum dot light emitting diode, wherein the quantum dot light emitting diode is an inverted quantum dot light emitting diode, and the quantum dot light emitting diode comprises an electron transport layer, the preparation method comprising the following steps:

Providing a substrate on which a cathode is disposed;

An electron transporting material is prepared on the cathode by the above-described method for producing an electron transporting material of the present invention to form the electron transporting layer.

Beneficial effect

The electron transporting material and the quantum dot light emitting diode of the invention utilize a high-low-value metal ion co-doped zinc oxide material, and the doped zinc oxide material is doped by a low-valent metal ion (divalent metal ion) , significantly increasing the doping limit of high-valent metal ions in the zinc oxide material, thereby significantly improving the electrical conductivity of zinc oxide as an electron transporting material, and finally improving the luminous efficiency of the quantum dot light emitting diode device; The doping of metal ions can be very selective. For example, divalent metal ions that conform to the conditions of easy oxidation and ionic radius and zinc ion can be used, and thus can be specifically used for electron transport materials in practical applications. Requires targeted selection with strong applicability and practicality.

The electron transporting material and the method for preparing the quantum dot light emitting diode provided by the invention adopts a partially oxidizable divalent metal ion doped zinc oxide material to be used as an electron transporting material, and the oxidation processing of the electron transporting material is used to obtain a high and low level. An electron transporting material in which a valence state of the same metal ion is co-doped with zinc oxide; the preparation method significantly improves the solid solubility limit of the high-valent metal ion in the zinc oxide material, and further optimizes the electrical conductivity of the zinc oxide as an electron transporting material, Furthermore, the luminous efficiency and device performance of the quantum dot light-emitting diode are improved as a whole; in the preparation method, only one metal ion needs to be doped to simultaneously improve the doping limit of the high-valent metal ion in the zinc oxide material and improve oxidation. The two functions of the conductive property of the zinc electron transporting material significantly save the synthesis cost of the doped zinc oxide nanocolloid solution, and the oxidation treatment process used in the invention is very simple, low in cost, easy to operate, and requires less equipment, and Reproducible.

DRAWINGS

1 is a schematic view showing the structure of a QLED prepared in Example 5 of the present invention.

Embodiments of the invention

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It is understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

In one aspect, an embodiment of the present invention provides an electron transporting material, wherein the electron transporting material is metal ion doped zinc oxide, and the metal ion is two or three metal ions of different valence states of the same metal element. And the lowest valence state of the metal ion is positive divalent.

The electron transporting material provided by the embodiment of the present invention utilizes the same metal element ion co-doped zinc oxide material in a high-price state, and the doped zinc oxide material is doped by a low-valent metal ion (divalent metal ion) The doping limit of the high-valent metal ions in the zinc oxide material is remarkably improved, thereby further improving the conductivity of the zinc oxide electron transporting material, and finally improving the luminous efficiency of the quantum dot light emitting diode device; The valence-doped metal ion can be widely selected, for example, a divalent metal ion which is easy to oxidize and has a ionic radius and a relatively close contact with zinc ions can be used, and thus can be used for an electron-transporting material according to practical applications. Specific requirements for targeted selection, with strong applicability and practicality.

Zinc oxide is a semiconductor material having a wurtzite structure. In the structure of zinc oxide, zinc ions are located at the center of a tetrahedron composed of oxygen ions, that is, each zinc ion is surrounded by four oxygen ions to form a coordination. A number of four cations. For zinc oxide materials, high-valent metal ions (Al ³⁺ , Ga ^{3+ ,} etc.) are doped into the wurtzite structure of zinc oxide, and high-valent metal ions replace the Zn ²⁺ sites, thereby generating free electrons. Increases the electron density, which in turn increases the electrical conductivity of the zinc oxide material. Therefore, this doping method is widely used to improve the electrical conductivity of zinc oxide materials. Theoretically, the higher the doping amount of high-valent ions, the higher the electrical conductivity of the zinc oxide material, but in fact, each doping ion has a doping limit in the host material. When the doping amount exceeds the doping limit, excess dopant ions will precipitate from the host material in the form of a second phase, which in turn has an unpredictable effect on the properties of the host material. One of the main factors determining the doping limit is the difference in ionic radius between the bulk ion and the dopant ion: the greater the difference in ionic radius between the two ions, the more the lattice distortion caused by ion doping Severe, the resulting kinetic instability causes the dopant ions to tend to precipitate out of the host material. Since the ionic radius of high-valence doping ions (Al ³⁺ , Ga ^{3+ ,} etc.) commonly used in industry is much smaller than the ionic radius of zinc ions (see Table 1 below), this difference exceeds 20% of the radius of zinc ions. Therefore, the solid solubility of these high-valent ions in the zinc oxide material is very limited.

Table 1

	Zn2+	Al3+	Ga3+	Mn4+	Mn2+
Ion radius (Å)	0.60	0.39	0.46	0.40	0.68
	Fe3+	Fe2+	Cr4+	Cr2+	Co4+	Co2+
Ion radius (Å)	0.49	0.64	0.41	0.71	0.40	0.67

In order to solve the problem that the high-valence ions have a small solid solubility in the zinc oxide material, the present invention selects some specific divalent metal ions (such as Fe ²⁺ , Mn ²⁺ , Cr ²⁺ , Co ^{2+ ,} etc.). Zinc oxide doping is performed. These divalent metal ions have two major characteristics: on the one hand, the ionic radii of these ions are very close to Zn ²⁺ and will be slightly larger than Zn ²⁺ (see Table 1), and can be heavily doped into the crystals of zinc oxide. On the other hand, these metal ions are very easily oxidized, and in an oxygen atmosphere, only relatively low temperatures are required to oxidize these metal ions to a higher valence state. By using these two characteristics, the embodiment of the present invention oxidizes a zinc oxide electron transport material doped with a plurality of specific divalent metal ions, so that some divalent metal ions are oxidized to a higher valence state, thereby forming the same species. A zinc oxide electron transporting material in which metal elements are co-doped with different valence ions. Here, although the ionic radius of the high-valence metal-doped ions formed after the oxidation treatment is still much smaller than the ionic radius of the zinc ions, the high-valence ion doping is caused by the existence of the low-cost co-doping of the same metal ions. The lattice distortion has become a certain relief, which in turn increases the doping limit of high-valent metal ions in zinc oxide. Therefore, by adjusting the doping percentage of the divalent metal ions and the parameters of the oxidation treatment process, the doping ratio of the high-valent metal ions in the zinc oxide material can be remarkably improved, and finally the zinc oxide electron transporting material having more excellent electrical conductivity can be obtained. .

In the embodiment of the present invention, a part of the divalent doped metal ion can be oxidized to a higher valence metal ion by doping the divalent metal element into the zinc oxide nanoparticle and performing heating oxidation. In the electron transporting material of the preferred embodiment of the present invention, the electron transporting material is zinc oxide co-doped including Fe ³⁺ and Fe ²⁺ ; or the electron transporting material is co-doped including Mn ⁴⁺ and Mn ²⁺ Zinc oxide; or the electron transporting material is zinc oxide co-doped with Co ⁴⁺ and Co ²⁺ ; or the electron transporting material is zinc oxide co-doped with Cr ⁴⁺ and Cr ²⁺ . Under certain conditions, the oxidation of the divalent metal ions is incomplete, possibly producing metal ions in the middle valence state. For example, when the electron transporting material is zinc oxide including Mn ⁴⁺ and Mn ²⁺ co-doping, a small amount of Mn ^{3 is} also included. ⁺ doping, that is, the electron transporting material is zinc oxide co-doped with Mn ⁴⁺ , Mn ²⁺ and Mn ³⁺ ; and the electron transporting material is zinc oxide co-doped with Co ⁴⁺ and Co ²⁺ And further comprising a small amount of Co ³⁺ doping, that is, the electron transporting material is Co ⁴⁺ , Co ²⁺ and Co ³⁺ co-doped zinc oxide; the electron transporting material comprises Cr ⁴⁺ and Cr ²⁺ When doped zinc oxide, a small amount of Cr ³⁺ doping is also included, that is, the electron transporting material is Cr ⁴⁺ , Cr ²⁺ and Cr ³⁺ co-doped zinc oxide.

Further, when the electron transport material of the present invention include Fe ²⁺ and Fe ³⁺ co-doped zinc oxide, the molar ratio of Fe ³⁺ and Fe ²⁺ is from 20: 1 to 1: 1; preferably 10:1-1:1; more preferably 4:1-5:4; still more preferably 9:5-3:2;

When the electron transport material including Cr ⁴⁺ and Cr ²⁺ co-doped zinc oxide, Cr ²⁺ and Cr ⁴⁺ molar ratio of 20: 1 to 1: 2, preferably 10: 1 to 1: 2 More preferably 3:1-2:3; still more preferably 3:2-2:3;

When the electron transporting material is zinc oxide co-doped including Mn ⁴⁺ and Mn ²⁺ , the molar ratio of Mn ^{4+ to} Mn ²⁺ is 10:1 to 2:3; preferably 5:1-2: 3; more preferably 2:1-1:1; still more preferably 3:2-1:1;

When the electron transport material include Co ²⁺ and Co ⁴⁺ co-doped zinc oxide, the molar ratio of Co ²⁺ and Co ⁴⁺ is 10: 1-2: 3; preferably 5: 1-2: 3; more preferably 2:1-1:1; still more preferably 3:2-1:1.

Correspondingly, a specific embodiment of the present invention provides a method for preparing an electron transporting material, comprising the following steps:

S01: providing a zinc salt and a divalent doped metal salt, dissolving the zinc salt and the divalent doped metal salt in a first solvent, and obtaining a divalent metal ion doped zinc oxide nanometer under alkaline conditions Granular solution;

S02: removing solvent and unreacted impurities in the divalent metal ion doped zinc oxide nanoparticle solution to obtain divalent metal ion doped zinc oxide nanoparticles;

S03: dissolving the divalent metal ion doped zinc oxide nanoparticles in a second solvent to obtain a divalent metal ion doped zinc oxide colloid solution;

S04: depositing the divalent metal ion doped zinc oxide colloid solution on a substrate and performing an oxidation treatment to obtain the electron transporting material.

The method for preparing an electron transporting material provided by the embodiment of the invention utilizes a partially oxidizable divalent metal ion doped zinc oxide material and is made into an electron transporting material, and the same metal element is obtained by oxidizing the electron transporting material. An electron transporting material in which different valence ions are co-doped with zinc oxide; the preparation method significantly improves the solid solubility limit of the high-valent metal ion in the zinc oxide material, and further optimizes the electrical conductivity of the zinc oxide electron transporting material, such an electron The transmission material can improve the luminous efficiency and device performance of the QLED device as a whole; in the preparation method, only one metal ion needs to be doped to simultaneously improve the doping limit of the high-valent metal ion in the zinc oxide material and improve the zinc oxide electron. The two functions of the conductive properties of the transmission material significantly save the synthesis cost of the doped zinc oxide nano-colloid solution, and the oxidation treatment process used in the embodiment of the invention is very simple, low in cost, easy to operate, and requires less equipment, and Reproducible. The preparation method can be carried out at room temperature, and is a low temperature solution method, and may be one of a low temperature alcoholysis method, a low temperature hydrolysis method, and the like.

Further, in step S01, the zinc salt is at least one selected from the group consisting of zinc acetate, zinc nitrate, zinc sulfate, zinc chloride, and hydrates of the above various zinc salts;

The divalent doped metal salt is at least one selected from the group consisting of a divalent iron salt, a divalent manganese salt, a divalent chromium salt, and a divalent cobalt salt; specifically, the divalent metal salt includes but is not limited to FeSO ₄ , FeCl ₂ , MnSO ₄ , MnCl ₂ , Mn(CH ₃ COO) ₂ , Mn(NO ₃ ) ₂ , Co(NO ₃ ) ₂ , Co(CH ₃ COO) ₂ , CoSO ₄ , CoCl ₂ , CrCl ₂ And one of the above hydrates of various metal salts.

The first solvent and the second solvent are independently selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol, ethylene glycol monomethyl ether, and dimethyl sulfoxide (DMSO). ;

Providing a zinc salt and a divalent doped metal salt, dissolving the zinc salt and the divalent doped metal salt in a first solvent, and obtaining a divalent metal ion doped zinc oxide nanoparticle solution under alkaline conditions . Dissolving the zinc salt and the divalent metal salt for the selected doping in a solvent at a room temperature, and simultaneously dissolving or diluting the reaction base in another identical or different solvent to obtain a lye Adding an alkali solution to the mixed solution of the zinc salt and the divalent metal salt to form a hydroxide intermediate, and then the polycondensation reaction of the hydroxide intermediate gradually forms a divalent metal ion doped zinc oxide nanoparticle. The temperature at which the alkali solution is added to carry out the reaction is 0-70 ° C; the time for adding the alkali solution to carry out the reaction is 0.5-4 h; specifically, the temperature of the mixed salt solution is first adjusted to 0-70 ° C, and the reaction temperature is selected. A range of 0-70 ° C to ensure the formation of doped zinc oxide nanoparticles and to obtain good particle dispersibility. When the reaction temperature is lower than 0 ° C, the reaction temperature is too low, which will significantly slow down the formation of zinc oxide nanoparticles, and even can not form zinc oxide nanoparticles, but only the hydroxide intermediate; and when the reaction temperature is higher than 70 ° C The obtained nanoparticles have poor dispersibility and agglomeration, which affects the late film formation of the doped zinc oxide colloidal solution. More preferably, the reaction temperature is selected from the range of room temperature to 50 ° C, and the reaction time is selected from 30 min to 4 h to ensure the formation of doped zinc oxide nanoparticles and control the particle size of the nanoparticles. When the reaction time is less than 30 min, the reaction time is too short, the formation of doped zinc oxide nanoparticles is insufficient, and the obtained nanoparticles have poor crystallinity; and when the reaction time exceeds 4 h, the excessively long particles grow up to generate The nanoparticles are too large and the particle size is not uniform, which affects the late film formation of the doped zinc oxide colloid solution. More preferably, the reaction time is selected to be 1~2h. The alkali liquid is selected from at least one of lithium hydroxide, sodium hydroxide, potassium hydroxide, tetramethylammonium hydroxide (TMAH), aqueous ammonia, ethanolamine, and ethylenediamine.

Further, the ratio of the sum of the amount of the substance of the zinc ion to the amount of the substance of the divalent metal ion and the amount of the substance of the OH ^- is 1: (1.5 - 2.5) by adding the alkali solution for reaction; to ensure doping Formation of zinc oxide nanoparticles and reduction of reaction by-product formation. When the hydroxide ion is too small, the metal salt is relatively excessively excessive, resulting in a large amount of metal salt unable to form doped zinc oxide nanoparticles; when the hydroxide ion is excessive, the excess hydroxide ion and the hydroxide intermediate form a stable The complex cannot be polycondensed to form doped zinc oxide nanoparticles. More preferably, the ratio of the amount of the substance of the zinc ion to the amount of the substance of the divalent metal ion and the amount of the substance of the OH ^- is 1: (1.7 - 1.9) and the alkali solution is added to carry out the reaction.

Further, in step S02, a precipitant may be added to the divalent metal ion doped zinc oxide nanoparticle solution to remove the solvent to obtain divalent metal ion doped zinc oxide nanoparticles. Preferably, the volume ratio of the precipitant to the divalent metal ion doped zinc oxide nanoparticle solution is (2-6):1; that is, the divalent metal ion doped zinc oxide nanoparticle solution after the end of the reaction A precipitant having a volume ratio of (2-6):1 was added thereto and then centrifuged to obtain divalent metal ion-doped zinc oxide nanoparticles. The obtained divalent metal ion doped zinc oxide nanoparticles are redissolved in the first solvent, and the washing process is repeated a plurality of times to remove the reactants that are not involved in the reaction. The finally obtained metal ion doped zinc oxide nanoparticles are dissolved in the second solvent to obtain a divalent metal ion doped zinc oxide colloid solution. The volume ratio of the precipitant to the divalent metal ion doped zinc oxide nanoparticle solution is selected to be (2-6):1 to ensure that no excessive precipitant occurs under the premature precipitation of the doped zinc oxide nanoparticles. Destroy the solubility of doped zinc oxide particles. More preferably, the volume ratio of the precipitant to the divalent metal ion doped zinc oxide nanoparticle solution is selected to be (3-5): 1; the precipitating agent is selected from the group consisting of ethyl acetate, n-hexane, n-heptane, acetone At least one of them.

Further, in step S03, in the divalent metal ion doped zinc oxide colloid solution, the doping concentration of the divalent metal is 2%-30%; that is, in step S01, the divalent metal is doped The amount of the metal ion doped substance in the salt is 2% to 30% by mole of the total metal ion, and the zinc salt and the divalent doped metal salt are dissolved in the first solvent (wherein The amount of the total metal ion substance refers to the sum of the amount of the metal ion doping substance and the amount of the zinc ion substance, thereby obtaining a divalent metal ion doped zinc oxide colloid solution, the second The doping concentration of the valence metal is 2% to 30%. The doping molar concentration of the divalent metal ion doping ions depends on the difference of the ionic radius of the divalent metal ion doping ion and the zinc ion: the smaller the ion radius difference, the doping of the divalent metal ion in the zinc oxide material The higher the limit, the higher the molar concentration of doping. Therefore, according to the difference in ionic radius of the doping ions and zinc ions of different divalent metal ions (see Table 1), the doping molar concentration of the divalent metal ions can be determined. The divalent metal ion doping ions include Fe ²⁺ , Mn ²⁺ , Cr ²⁺ , Co ^{2+ ,} etc. which are easily oxidized to a higher valence state, specifically, the doping molar concentration of Fe ²⁺ is 5%. -30%; the molar concentration of Co ²⁺ is 2%-25%; the molar concentration of Mn ²⁺ is 2%-25%; the molar concentration of Cr ²⁺ is 2%-20%. The doping molar concentration of the divalent metal ion doping ions does not exceed the doping limit, and the doping of the high-valent metal ions in the zinc oxide after the oxidation treatment can be improved only after reaching a certain doping molar concentration. The limit further realizes the effect of further improving the electrical conductivity of the zinc oxide electron transporting material. Therefore, more preferably, the molar concentration of Fe ²⁺ is 7%-27%; the molar concentration of Co ²⁺ is 4%-23%; the molar concentration of Mn ²⁺ is 4%-23% The molar concentration of Cr ²⁺ is 4%-18%. Further, more preferably, the doping molar concentration of Fe ²⁺ is 10%-25%; the molar concentration of Co ²⁺ is 8%-20%; the molar concentration of Mn ²⁺ is 8%-20% The molar concentration of Cr ²⁺ is 8%-15%. Optimum, the molar concentration of Fe ²⁺ is 14%-22%; the molar concentration of Co ²⁺ is 12%-18%; the molar concentration of Mn ²⁺ is 12%-18%; Cr The molar concentration of ²⁺ is 10% to 14%. At the optimal metal ion doping molar concentration, the presence of a sufficient amount of low-valent metal ions can fully exert the effect of lowering the lattice distortion caused by high-valence metal ions, and maximize the high-valence metal ions in the zinc oxide material. In the doping limit, a sufficient amount of high-valent metal ions are doped into the zinc oxide crystal structure to maximize the conductivity of the zinc oxide material.

Further, a deposition method for depositing the divalent metal ion doped zinc oxide colloid solution on a substrate includes, but is not limited to, spin coating, blade coating, printing, spray coating, roll coating, electrodeposition One of the laws and the like is then subjected to an oxidation treatment to produce the zinc oxide electron transporting material.

Further, in step S04, the divalent metal ion doped zinc oxide colloid solution is deposited on the substrate, and the step of performing the oxidation treatment comprises: depositing the divalent metal ion doped zinc oxide colloid solution on the substrate. The substrate is subjected to heat treatment in an oxygen atmosphere at a heat treatment temperature of 100 to 400 ° C and a heat treatment time of 0.5 to 12 hours. Among them, the specific conditions in the oxygen environment are: the oxygen used is industrial grade pure oxygen with a purity of 99.5% or more. The conditions of the oxidation treatment may be different depending on the degree of easy oxidation of the divalent doped metal ions in oxygen (relatively, Fe ²⁺ and Cr ^{2+ are} most easily oxidized under an oxygen atmosphere and the same heating temperature, and secondly It is Mn ²⁺ and the slowest is Co ²⁺ ) for further optimization. Therefore, more preferably, the heat treatment temperature of the Fe ²⁺ -doped zinc oxide electron transport material is 100-300 ° C, the heat treatment time is 30 min-6 h; the heat treatment temperature of the Cr ²⁺ -doped zinc oxide electron transport material is 100 -300 ° C, heat treatment time is 30min-6h; Mn ²⁺ doped zinc oxide electron transport material heat treatment temperature is 200-400 ° C, heat treatment time is 1-8h; Co ²⁺ doped zinc oxide electron transport material The heat treatment temperature is 300-400 ° C, and the heat treatment time is 4 h-12 h.

During the oxidation treatment, the oxidation treatment temperature is selected to ensure the formation of high-valent metal ions and to prevent damage to the substrate. When the oxidation treatment temperature is too low, the conversion process of the divalent metal ion doping ions to the high-valent state metal ions will become very slow, even making the oxidation process impossible; and when the oxidation treatment temperature is too high, it is possible The substrate of the electron transporting material is destroyed, and the oxidation process of the divalent metal ion doping ions is too fast to be controlled.

The key point of the embodiment of the present invention is that after the oxidation treatment of the doped zinc oxide electron transporting material, a sufficient amount of divalent metal ions are preserved to increase the doping limit of the high-valent metal ions in the zinc oxide material, and Sufficient high-valence doping metal ions are formed and doped into the zinc oxide crystal structure. The oxidation treatment time in the oxidation treatment mode is the key to regulating the doping ratio of the high-low-state metal ions in the zinc oxide electron transport material. When the oxidation treatment time is too short, the amount of divalent doped metal ions oxidized to high-valent doped metal ions is too small to improve the conductivity of the zinc oxide electron transport material; and when the oxidation treatment time is too long, Excessive high-valence doping metal ions are generated, resulting in too little residual divalent-doped metal ions, which cannot improve the doping limit of high-valent metal ions in zinc oxide, and thus partially generated high-valence metal ions will The form of the second phase is precipitated from the zinc oxide material, which adversely affects the electrical conductivity of the zinc oxide electron transport material.

When the nano zinc oxide electron transporting material of the present invention is Fe ³⁺ and Fe ²⁺ co-doped zinc oxide, the molar ratio of Fe ^{3+ to} Fe ²⁺ is 20:1 to 1:1; preferably 10: 1-1:1; more preferably 4:1-5:4; still more preferably 9:5-3:2;

When the electron transport material including Cr ⁴⁺ and Cr ²⁺ co-doped zinc oxide, Cr ²⁺ and Cr ⁴⁺ molar ratio of 20: 1 to 1: 2, preferably 10: 1 to 1: 2 More preferably 3:1-2:3; still more preferably 3:2-2:3;

When the electron transporting material is zinc oxide co-doped including Mn ⁴⁺ and Mn ²⁺ , the molar ratio of Mn ^{4+ to} Mn ²⁺ is 10:1 to 2:3; preferably 5:1-2: 3; more preferably 2:1-1:1; still more preferably 3:2-1:1;

When the electron transport material include Co ²⁺ and Co ⁴⁺ co-doped zinc oxide, the molar ratio of Co ²⁺ and Co ⁴⁺ is 10: 1-2: 3; preferably 5: 1-2: 3; more preferably 2:1-1:1; still more preferably 3:2-1:1.

Embodiments of the present invention also provide a quantum dot light emitting diode comprising an anode, a cathode, and a quantum dot emitting layer between the anode and the cathode, and an electron transport is disposed between the cathode and the quantum dot emitting layer The layer, the electron transport layer is composed of the above-described electron transport material of the embodiment of the present invention.

Correspondingly, a method for preparing a quantum dot light emitting diode, wherein the quantum dot light emitting diode is an inverted quantum dot light emitting diode, and the quantum dot light emitting diode comprises an electron transport layer, the preparation method comprising the following steps:

Providing a substrate on which a cathode is disposed;

An electron transporting material is prepared on the cathode by the above-described method for producing an electron transporting material according to an embodiment of the present invention to form the electron transporting layer.

The preparation method of the quantum dot light-emitting diode is made by using a partially oxidizable divalent metal ion doped zinc oxide material to form an electron transporting material, and by oxidizing the electron transporting material, a high-low-valent state metal ion is obtained. The electron transport material layer doped with zinc oxide; the preparation method significantly improves the solid solubility limit of the high-valent metal ion in the zinc oxide material, further optimizes the electrical conductivity of the zinc oxide electron transport material, and thereby improves the overall illumination of the device. Efficiency and device performance.

In a specific embodiment, a method for preparing a QLED device includes:

A: firstly depositing a doped zinc oxide colloid solution on the cathode substrate, and performing an oxidation treatment to obtain the high-low-value homogenous doped zinc ion electron transport layer of the same metal ion;

B: depositing a quantum dot luminescent layer on the electron transport layer;

C: Finally, a hole transport layer is deposited on the quantum dot light-emitting layer, and the anode is vapor-deposited on the hole transport layer to obtain a quantum dot light-emitting diode.

The substrate material of the present invention is a glass piece, and the cathode is an ITO substrate.

Further, a deposition method for depositing the divalent metal ion doped zinc oxide colloid solution on a substrate includes, but is not limited to, spin coating, blade coating, printing, spray coating, roll coating, electrodeposition a method of preparing a doped zinc oxide electron transport layer having a thickness of 10 to 100 nm, and when the thickness of the electron transport layer is less than 10 nm, the film layer is very It is easy to be broken down by electrons, and the injection performance of carriers cannot be guaranteed. When the thickness of the electron transport layer is greater than 100 nm, electron injection is hindered and the charge injection balance of the device is affected.

In the method for preparing a QLED device, the quantum dots of the quantum dot light-emitting layer are one of red, green, and blue. Can be CdS, CdSe, CdTe, ZnO, ZnS, ZnSe, ZnTe, GaAs, GaP, GaSb, HgS, HgSe, HgTe, InAs, InP, InSb, AlAs, AlP, CuInS, CuInSe, and various core-shell quantum dots Or at least one of the alloy structure quantum dots. It can be any of the common red, green and blue quantum dots or other yellow light. The quantum dots in this step can be cadmium-containing or cadmium-free. The quantum dot luminescent layer material has the characteristics of wide excitation spectrum and continuous distribution, and high stability of emission spectrum. Preparation of quantum dot luminescent layer: The substrate which has been spin-coated with the electron transport layer is placed on the homogenizing machine, and a solution of a certain concentration of the luminescent substance is spin-coated to form a film, and the concentration of the solution, the spin coating speed and the spin coating time are adjusted. To control the thickness of the luminescent layer, about 20~60 Nm, dry at the appropriate temperature.

In the QLED device manufacturing method, the hole transport layer may be made of a hole transporting material conventional in the art, including but not limited to TFB, PVK, Poly-TPD, TCTA, CBP, etc. or a mixture of any combination thereof. It can also be other high performance hole transport materials. Hole transport layer: the substrate on which the quantum dot light-emitting layer has been spin-coated is placed on a homogenizer, and spin-coated with a solution of the prepared hole transport material; by adjusting the concentration of the solution, the spin coating speed and spin coating Time to control the film thickness and then dry at the appropriate temperature.

Next, the substrate in which the functional layers are deposited is placed in a vapor deposition chamber, and a 15-30 nm metal silver or aluminum is thermally evaporated through the mask as an anode, or a nano-Ag wire or a Cu wire is used, which has a small The resistor allows the carriers to be injected smoothly.

Further, the obtained QLED is subjected to a packaging process, and the package process may be a common machine package or a manual package. Preferably, in the environment of the encapsulation treatment, the oxygen content and the water content are both lower than 0. .1ppm to ensure the stability of the device.

The present invention has been subjected to a number of tests in succession, and a part of the test results are now described in further detail as a reference, and will be described in detail below in conjunction with specific embodiments.

Example 1

The synthesis of iron ion doped zinc oxide colloidal solution, the preparation of iron ion doped zinc oxide electron transport material and its oxidation treatment process are described in detail below by taking iron ion doped zinc oxide electron transport material as an example.

First, an appropriate amount of zinc acetate and a ferric salt FeSO _{4 were} added to 50 ml of a methanol solvent to form a mixed salt solution having a total concentration of 0.1 mol/L, wherein the Fe ²⁺ molar concentration was 20%. At the same time, an appropriate amount of potassium hydroxide powder was dissolved in another 50 ml of methanol solvent to form a lye having a concentration of 0.3 mol/L. The mixed salt solution was then heated to 50 ° C, and the potassium hydroxide solution was added dropwise until the molar ratio of hydroxide ions to metal ions was 1.7:1. After the completion of the dropwise addition of the potassium hydroxide solution, the mixed solution was further stirred at 50 ° C for 2 hours to obtain a homogeneous transparent solution. Subsequently, a 3:1 volume ratio of ethyl acetate solvent was added to the homogeneous clear solution to produce a large amount of white precipitate in the clear solution. The cloudy solution was centrifuged at 7000 rpm, and the resulting white precipitate was again dissolved in a methanol solvent. This cleaning process is repeated four times. The finally obtained white precipitate was dissolved in an appropriate amount of ethanol solvent to obtain a divalent iron ion-doped zinc oxide colloid solution having a concentration of 30 mg/ml.

The obtained 30 mg/ml divalent iron ion doped zinc oxide colloid solution was deposited on the ITO substrate by spin coating. The spin coating speed is 3000 rpm, and the spin coating time is 30 s to control the thickness of the doped zinc oxide electron transport material to be about 50 nm.

Finally, the substrate on which the divalent iron ion-doped zinc oxide electron transport material is deposited is placed in a muffle furnace. The zinc oxide electron transporting material was heated to 200 ° C in an oxygen atmosphere with a purity of 99.5%, and kept at this temperature for 2 h to obtain a zinc oxide electron transporting material co-doped with high-and low-valent iron ions.

Example 2

The synthesis of chromium ion doped zinc oxide colloidal solution, the preparation of chromium ion doped zinc oxide electron transport material and its oxidation treatment process are described in detail below by taking chromium ion doped zinc oxide electron transport material as an example.

First, an appropriate amount of zinc nitrate and a divalent chromium salt CrCl _{2 were} added to 50 ml of an ethanol solvent to form a mixed salt solution having a total concentration of 0.1 mol/L, wherein the molar concentration of Cr2+ was 15%. At the same time, an appropriate amount of lithium hydroxide powder was dissolved in another 50 ml of ethanol solvent to form a lye having a concentration of 0.2 mol/L. The mixed salt solution was then heated to 40 ° C and the lithium hydroxide solution was added dropwise until the molar ratio of hydroxide ions to metal ions was 1.9:1. After the completion of the dropwise addition of the lithium hydroxide solution, the mixed solution was further stirred at 30 ° C for 1 h to obtain a uniform transparent solution. Subsequently, a 4:1 by volume heptane solvent was added to the homogeneous clear solution to produce a large amount of white precipitate in the clear solution. The cloudy solution was centrifuged at 7000 rpm, and the resulting white precipitate was again dissolved in an ethanol solvent. This cleaning process is repeated four times. The finally obtained white precipitate was dissolved in an appropriate amount of an ethanol solvent to obtain a divalent chromium ion-doped zinc oxide colloidal solution having a concentration of 30 mg/ml.

The obtained 30 mg/ml divalent chromium ion doped zinc oxide colloid solution was deposited on the ITO substrate by spin coating. The spin coating speed is 1500 rpm, and the spin coating time is 30 s to control the thickness of the doped zinc oxide electron transport material to be about 80 nm.

Finally, a substrate on which a divalent chromium ion-doped zinc oxide electron transport material is deposited is placed in a muffle furnace. The zinc oxide electron transport material was heated to 200 ° C in an oxygen atmosphere with a purity of 99.5%, and was kept at this temperature for 4 h to obtain a zinc oxide electron transport material co-doped with high-valent chromium ions.

Example 3

The synthesis of manganese ion doped zinc oxide colloidal solution, the preparation of manganese ion doped zinc oxide electron transport material and its oxidation treatment process are described in detail below by taking manganese ion doped zinc oxide electron transport material as an example.

First, an appropriate amount of zinc sulfate and a manganese salt Mn(CH ₃ COO) _{2 were} added to 50 ml of DMSO solvent to form a mixed salt solution having a total concentration of 0.1 mol/L, wherein the molar concentration of Mn ²⁺ was 15%. At the same time, an appropriate amount of TMAH powder was dissolved in another 30 ml of ethanol solvent to form a lye having a concentration of 0.3 mol/L. The mixed salt solution was then added dropwise to the TMAH solution at room temperature until the molar ratio of hydroxide ions to metal ions was 1.5:1. After the completion of the dropwise addition of the TMAH solution, the mixed solution was further stirred at room temperature for 2 hours to obtain a uniform transparent solution. Subsequently, a 4:1 volume ratio of ethyl acetate solvent was added to the homogeneous clear solution to produce a large amount of white precipitate in the clear solution. The cloudy solution was centrifuged at 7000 rpm, and the resulting white precipitate was again dissolved in an ethanol solvent. This cleaning process is repeated four times. The finally obtained white precipitate was dissolved in an appropriate amount of an ethanol solvent to obtain a divalent manganese ion-doped zinc oxide colloidal solution having a concentration of 30 mg/ml.

The obtained 30 mg/ml divalent manganese ion doped zinc oxide colloid solution was deposited on the ITO substrate by spin coating. The spin coating speed was 4500 rpm and the spin coating time was 30 s to control the thickness of the doped zinc oxide electron transport material to be about 20 nm.

Finally, a substrate on which a divalent manganese ion-doped zinc oxide electron transport material is deposited is placed in a muffle furnace. The doped zinc oxide electron transport material was heated to 300 ° C in an oxygen atmosphere with a purity of 99.5%, and kept at this temperature for 6 h to obtain a zinc oxide electron transport material co-doped with high-value manganese ions.

Example 4

The synthesis of cobalt ion doped zinc oxide colloidal solution, the preparation of cobalt ion doped zinc oxide electron transport material and its oxidation treatment process are described in detail below by taking cobalt ion doped zinc oxide electron transport material as an example.

First, an appropriate amount of zinc chloride and a divalent cobalt salt Co(NO ₃ ) _{2 were} added to 50 ml of a methanol solvent to form a mixed salt solution having a total concentration of 0.1 mol/L, wherein the molar concentration of Co ²⁺ was 15%. At the same time, an appropriate amount of sodium hydroxide powder was dissolved in another 50 ml of methanol solvent to form a lye having a concentration of 0.3 mol/L. The mixed salt solution was then heated to 50 ° C, and the sodium hydroxide solution was added dropwise until the molar ratio of hydroxide ions to metal ions was 1.8:1. After the completion of the dropwise addition of the sodium hydroxide solution, the mixed solution was further stirred at 50 ° C for 1 h to obtain a uniform transparent solution. Subsequently, a n: hexane solvent in a volume ratio of 5:1 was added to the homogeneous clear solution to produce a large amount of white precipitate in the clear solution. The cloudy solution was centrifuged at 7000 rpm, and the resulting white precipitate was again dissolved in a methanol solvent. This cleaning process is repeated four times. The finally obtained white precipitate was dissolved in an appropriate amount of an ethanol solvent to obtain a divalent cobalt ion-doped zinc oxide colloid solution having a concentration of 30 mg/ml.

The obtained 30 mg/ml divalent cobalt ion doped zinc oxide colloid solution was deposited on the ITO substrate by spin coating. The spin coating speed is 3000 rpm, and the spin coating time is 30 s to control the thickness of the doped zinc oxide electron transport material to be about 50 nm.

Finally, a substrate on which a divalent cobalt ion-doped zinc oxide electron transport material is deposited is placed in a muffle furnace. The zinc oxide electron transporting material was heated to 400 ° C in an oxygen atmosphere with a purity of 99.5%, and was kept at this temperature for 8 h to obtain a zinc oxide electron transporting material co-doped with high and low cobalt ions.

Example 5

The method for preparing the QLED device comprises the following steps:

A: firstly spin-coating a doped zinc oxide electron transport layer on an ITO substrate and oxidizing;

B: depositing a quantum dot luminescent layer on the electron transport layer;

C: Finally, a hole transport layer is deposited on the quantum dot light-emitting layer, and the anode is vapor-deposited on the electron transport layer to obtain a quantum dot light-emitting diode.

The QLED device of this embodiment is of an inverted configuration. FIG. 1 is a schematic structural diagram of a QLED device. The QLED device includes a substrate 1, a cathode 2, an electron transport layer 3, a quantum dot light-emitting layer 4, and an empty layer from bottom to top. Hole transport layer 5, anode 6. The material of the substrate 1 is a glass piece, the material of the cathode 2 is an ITO substrate, and the material of the electron transport layer 3 is any high-low-value metal ion co-doped zinc oxide of any of the embodiments 1-4. The material of the hole transport layer 5 is TFB, and the material of the anode 6 is Al.

The above is only the preferred embodiment of the present invention, and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection of the present invention. Within the scope.

Claims (15)

1. An electron transporting material, characterized in that the electron transporting material is metal ion doped zinc oxide, the metal ion is two or three metal ions of different valence states of the same metal element, and the metal The lowest valence state of ions is positive divalent.
2. The electron transporting material according to claim 1, wherein said electron transporting material is zinc oxide co-doped including Fe ³⁺ and Fe ²⁺ ;

   The electron transporting material is zinc oxide co-doped including Mn ⁴⁺ and Mn ²⁺ ; or

   The electron transporting material is zinc oxide co-doped including Co ⁴⁺ and Co ²⁺ ; or

   The electron transporting material is zinc oxide co-doped including Cr ⁴⁺ and Cr ²⁺ .
3. Said electron transport material as claimed in claim 2, wherein the electron transport material include Fe ²⁺ and Fe ³⁺ co-doped zinc oxide, the electron transport material, and Fe ³⁺ of Fe ²⁺ The molar ratio is 20:1-1:1; or,

   The electron transporting material is zinc oxide co-doped including Mn ⁴⁺ and Mn ²⁺ , and the molar ratio of Mn ^{4+ to} Mn ²⁺ in the electron transporting material is 10:1 to 2:3;

   The electron transport material include Co ²⁺ and Co ⁴⁺ co-doped zinc oxide, the molar ratio of the electron transporting material in Co ²⁺ and Co ⁴⁺ is 10: 1-2: 3; or

   The electron transporting material is zinc oxide co-doped including Cr ⁴⁺ and Cr ²⁺ , and the molar ratio of Cr ^{4+ to} Cr ²⁺ in the electron transporting material is 20:1 to 1:2.
4. Said electron transport material as claimed in claim 3, wherein the electron transport material include Fe ²⁺ and Fe ³⁺ co-doped zinc oxide, the electron transport material, and Fe ³⁺ of Fe ²⁺ The molar ratio is 9:5-3:2; or,

   The electron transporting material is zinc oxide co-doped including Mn ⁴⁺ and Mn ²⁺ , and the molar ratio of Mn ^{4+ to} Mn ²⁺ in the electron transporting material is 3:2-1:1; or

   The electron transport material include Co ²⁺ and Co ⁴⁺ co-doped zinc oxide, the electron transport material, and Co ⁴⁺ molar ratio of Co ²⁺ to 3: 2-1: 1; or

   The electron transporting material is zinc oxide co-doped including Cr ⁴⁺ and Cr ²⁺ , and the molar ratio of Cr ^{4+ to} Cr ²⁺ in the electron transporting material is 3:2 to 2:3.
5. The electron transporting material according to claim 2, wherein said electron transporting material is Mn ⁴⁺ , Mn ²⁺ and Mn ³⁺ co-doped zinc oxide; or

   The electron transporting material is Co ⁴⁺ , Co ²⁺ and Co ³⁺ co-doped zinc oxide; or

   The electron transporting material is Cr ⁴⁺ , Cr ²⁺ and Cr ³⁺ co-doped zinc oxide.
6. A method for preparing an electron transporting material, comprising the steps of:

   Providing a zinc salt and a divalent doped metal salt, dissolving the zinc salt and the divalent doped metal salt in a first solvent, and obtaining divalent metal ion doped zinc oxide nanoparticles under alkaline conditions;

   Dispersing the divalent metal ion doped zinc oxide nanoparticles in a second solvent to obtain a divalent metal ion doped zinc oxide colloid solution;

   Depositing the divalent metal ion doped zinc oxide colloid solution on the substrate and performing an oxidation treatment to obtain the electron transporting material doped with metal ions, wherein the metal ions are two different valence states of the same metal element Or three metal ions, and the lowest valence state of the metal ions is positive divalent.
7. The method of preparing an electron transporting material according to claim 6, wherein the zinc salt is selected from the group consisting of zinc acetate and a hydrate thereof, zinc nitrate and a hydrate thereof, zinc sulfate and a hydrate thereof, and zinc chloride and At least one of hydrates; and/or,

   The divalent doped metal salt is selected from at least one of a divalent iron salt, a divalent manganese salt, a divalent chromium salt, and a divalent cobalt salt; and/or,

   The first solvent and the second solvent are independently selected from at least one of water, methanol, ethanol, propanol, butanol, ethylene glycol, ethylene glycol monomethyl ether, and dimethyl sulfoxide.
8. The method for preparing an electron transporting material according to claim 6, wherein the amount of the metal ion doping substance in the divalent doping metal salt is 2% by mole based on the total metal ion species. %, the zinc salt and the divalent doped metal salt are dissolved in the first solvent; wherein the amount of the total metal ion substance refers to the amount of the metal ion doping substance and the zinc ion substance The sum of the quantities.
9. The method for preparing an electron transporting material according to claim 8, wherein the divalent doped metal salt is a divalent iron salt, and in the divalent iron salt, the amount of Fe ²⁺ is in the total metal ion. a molar percentage of the amount of the substance is from 5% to 30%, and the zinc salt and the divalent iron salt are dissolved in the first solvent;

   Alternatively, the divalent doped metal salt is a divalent manganese salt, and in the divalent manganese salt, the molar percentage of the amount of the substance of the Mn ^{2+ to} the total metal ion is 2% to 25%. Said zinc salt and said divalent manganese salt are dissolved in the first solvent;

   Alternatively, the divalent doped metal salt is a divalent cobalt salt, and in the divalent cobalt salt, the molar percentage of the amount of the substance of Co ^{2+ to} the total metal ion is 2% to 25%. Said zinc salt and said divalent cobalt salt are dissolved in the first solvent;

   Alternatively, the divalent doped metal salt is a divalent chromium salt, and in the divalent chromium salt, the molar percentage of the amount of the substance of Cr ^{2+ to} the total metal ion is 2% to 20%. The zinc salt and the divalent chromium salt are dissolved in the first solvent.
10. The method for preparing an electron transporting material according to claim 9, wherein the divalent doped metal salt is a divalent iron salt, and in the divalent iron salt, the amount of Fe ²⁺ is in the total metal ion. a molar percentage of the amount of the substance is from 14% to 22%, and the zinc salt and the divalent iron salt are dissolved in the first solvent;

    Alternatively, the divalent doped metal salt is a divalent manganese salt, and in the divalent manganese salt, the molar percentage of the amount of the substance of the Mn ^{2+ to} the total metal ion is 12% to 18%. Said zinc salt and said divalent manganese salt are dissolved in the first solvent;

    Alternatively, the divalent doped metal salt is a divalent cobalt salt, and in the divalent cobalt salt, the molar percentage of the amount of the substance of Co ^{2+ to} the total metal ion is 12% to 18%. Said zinc salt and said divalent cobalt salt are dissolved in the first solvent;

    Alternatively, the divalent doped metal salt is a divalent chromium salt, and in the divalent chromium salt, the molar percentage of the amount of the substance of Cr ^{2+ to} the total metal ion is 10% to 14%. The zinc salt and the divalent chromium salt are dissolved in the first solvent.
11. The method of producing an electron transporting material according to claim 6, wherein the ratio of the amount of the substance of the zinc ion to the amount of the substance of the doped divalent metal ion and the amount of the substance of the OH ^- is 1 : (1.5-2.5), the zinc salt and the divalent doped metal salt are dissolved in a first solvent to obtain a divalent metal ion doped zinc oxide nanoparticle solution under basic conditions.
12. A method of producing an electron transporting material according to claim 6, wherein

    Depositing the divalent metal ion doped zinc oxide colloid solution on the substrate, and performing the oxidizing treatment comprises: depositing the divalent metal ion doped zinc oxide colloid solution on the substrate, in an oxygen environment, The substrate is subjected to heat treatment.
13. The method for preparing an electron transporting material according to claim 12, wherein the divalent doped metal salt is a divalent iron salt, the heat treatment temperature is 100-300 ° C, and the heat treatment time is 0.5-6 h;

    Or the divalent doped metal salt is a divalent manganese salt, the heat treatment temperature is 200-400 ° C, and the heat treatment time is 1-8 h;

    Or the divalent doped metal salt is a divalent cobalt salt, the heat treatment temperature is 300-400 ° C, and the heat treatment time is 4-12 h;

    Alternatively, the divalent doped metal salt is a divalent chromium salt, the heat treatment temperature is 100-300 ° C, and the heat treatment time is 0.5-6 h.
14. A quantum dot light emitting diode comprising an anode, a cathode and a quantum dot luminescent layer between the anode and the cathode, an electron transport layer disposed between the cathode and the quantum dot luminescent layer, the electron transport The layer consists of the electron transporting material of any of claims 1-5.
15. A method for preparing a quantum dot light emitting diode, wherein the quantum dot light emitting diode is an inverted quantum dot light emitting diode, and the quantum dot light emitting diode comprises an electron transport layer, wherein the preparation method comprises the following steps:

    Providing a substrate on which a cathode is disposed;

    An electron transporting material is prepared on the cathode by the preparation method according to any one of claims 6 to 13 to form the electron transporting layer.